Microwave modulator



Aug. 12, 1958 J. F. ZALESKI MICROWAVE MODULATOR 2 Sheets-Sheet 1 FiledAug. 9, 1956 r O t "I On D n 3 1958 .J. F. ZALESKI 2,847,647

MICROWAVE MODULATOR Filed Aug. 9, 1956 2 Sheets-"Sheet 2 Inventor OIM/I? 2/91565/ United States Patent MICROWAVE MODULATOR John F. Zaleski,Valhalla, N. Y., assignor to General Precision Laboratory Incorporated,a corporation of New York Application August 9, 1956, Serial No. 602,974

7 Claims. (Cl. 332-17) This invention relates to devices for modulatingmicrowave energy in waveguides and more especially to ferrite modulatorswhich operate to vary Waveguide impedance and which are effective atvery high modulation frequencies.

The conventional microwave modulator employing a crystal diode islimited in power capacity, is non-linear at all but very small powers,and is inherently noisy. For these reasons such a modulator leavessomething to be desired in fulfilling the purposes of this invention, atboth low and high powers. On the other hand, the ferrite rotatoremployed as a modulator is inherently quiet. It is capable of fairlyhigh power modulation, and when employed as taught herein can be usedfor very high frequency modulation.

Ferrite elements employed in microwave field rotators and modulators areusually composed of polyoxides of two, three or more metals such asiron, manganese, magnesium, copper, nickel, cobalt and zinc. Suchferrites are described by E. Albers-Schoenberg in the Journal of AppliedPhysics, volume 25, No. 2, on pages 152-154. Their use to rotate theplane of polarization of a micro wave field is fully described in PatentNo. 2,644,930 dated July 7, 1953, and Patent No. 2,719,274 datedSeptember 27,1955. It is not, therefore, necessary to describe thisdevice further. However, for the purposes of this invention, it shouldbe understood that the ferrite properties of high high-frequencyresistance and high permeability, and the peculiar property of rotatinga polarized guided microwave field are important.

As described in Patent No. 2,719,274 one method of employing ferrite torotate a microwave field consists of positioning a ferrite rod axiallyin a round waveguide which can transmit a microwave field in a radiallynonsymmetrical mode such as the TE mode. The rod is magnetized by directcurrent passed through a coil coaxially positioned around the outside ofthe waveguide at the position of the rod, and the direction of themicrowave field polarization is rotated in amount and sense dependingupon the amount and sense of ferrite magnetic induction, among otherfactors, and independent of the magnitude or direction of propogation ofthe microwave energy. When this arrangement is employed as a modulatorthe direct current excitation is replaced by alternating or varyingmodulating current excitation, and the waveguide is arranged with anoutlet which then emits microwave energy having its amplitude, andusually its phase also, varied in accordance with the modulatingcurrent.

This modulating arrangement can be employed with modulating frequenciesin the kilocycle range or even up to about one megacycle, but is notsatisfactory with increasing modulating frequency. This is because themetallic waveguide absorbs increasing amounts of power from themagnetizing solenoid as the frequency is increased, and at frequenciesabove, say, five mc. p. s. it serves as a virtually complete shieldbetween the magnetizing coil and the ferrite rod. It is not desirable toPatented Aug. 12, 1958 insert the magnetizing coil inside the waveguidearound the ferrite rod because the metal of the solenoid would greatlyinterfere with the propagation of the microwave field energy through thewaveguide. This is particularly true because at high frequencies, suchas at 20 mc. p. s.,

the solenoid takes the form of a single-layer helix having widely spacedturns and, if carrying large amounts of power, is made of heavy wire orrod. Such a mass of metal within the waveguide would cause a microwaveimpedance discontinuity so large as not to be tolerated. Moreover theeffect on the modulating circuit of the capacitance of the waveguidewalls would be highly undesirable because it would shunt most of themodulating energy to ground.

For these and other reasons the instant invention extends the ferriterod outside the waveguide and applies the modulating current coil tothat portion of the rod which extends outside the waveguide. Themodulating current magnetizes the ferrite rod, the magnetic inductionfollowing the modulating current amplitude and extending along the rodto that portion inside the waveguide. The resulting modulated magneticinduction field within the waveguide interacts with the microwave fieldto rotate its polarization direction.

When the ferrite rod is employed in a rectangular waveguide the fieldrotation force cannot cause rotation of the direction of polarizationaway from the single direction permitted by the waveguide, but doescause a change in amplitude of the field, with a consequent change inthe effective waveguide impedance.

In employing a ferrite rod extending through the Waveguide wall with amodulating coil around the external end, the waveguide wall wouldconstitute a single closed turn about the ferrite rod which at highfrequencies would absorb all of the modulating power and wouldcompletely prevent any modulation whatever of the magnetic inductionwithin the Waveguide. It is a feature of the invention therefore tosplit the waveguide wall so that it does not form a closed turn aboutthe ferrite rod. In some forms of the invention this split or slot isinherently non-radiating, while in other forms the slot must beprotected by a wavetrap in the manner well understood to prevent leakageof microwave energy out of the waveguide. When the modulator is thusconstructed the frequency of the modulating current is in no Way limitedby the construction of the modulator. Specifically, the frequency of themodulating current may range up to and beyond 500 me. p. s., a value somuch above the limits of all existing microwave modulators as toindicate that the instant modulator is qualitatively different frompreviously existing modulators.

The purpose of this invention, then, is to provide an improved microwavemodulator for both low and high modulation frequencies.

Another purpose of this invention is to provide a microwave modulatorcomprising a waveguide containing a longitudinal plane of the maximumelectric field so as I not to load the magnetic induction field.

Still another purpose of this invention is to provide a modulatorcomprising a microwave guide having a ferrite rod passing diagonallythrough at least one side wall of the waveguide which is split toprevent its form- ,t1tWindmill secured from the detailed description andaccompanying.

drawings, in which:

Figure l'is an isometridsketch, partly schematic, of.

one embodiment of the invention e ploying a 3 'db coupler.

Figure 2 schematically illustrates part of the operation of the devicedepicted in Fig. 1.

Figure 3 depicts another embodiment of the invention.

Figures 4 and'5 illustrate design details of the embodiment depicted inFig. 3.

Referring now to Fig. 1, two rectangular microwave hollow waveguides-11and 12 are positioned parallel with each' otherv so as j-to have onecommon narrow side 13. The right ends-of the-waveguides are closed andshortcircuited by a conductive metal plate 14. The common narrow side 13is provided with a gap, window, or discontinuity, the edges thereofbeing designated 16 and 17. The length of this gap has such dimensionthat the twoiwaveguides so connected constitute a three-decibeldirectional coupler. This well-known construction is described by H. J.Riblet in Proceedings of the Institute of Radio Engineers for February1952, entitled The Short-SlotHybrid Junction. The left ends of thewaveguides are depicted as open, it being understood that in operationeither one of them, as for example, end 18 of the right waveguide asshown, is connected to a source of microwave power, and end 19 of theleft waveguide constitutes the output of the microwave modulator, andisto beconnected to utilizationapparatus.

The directional coupler as so far described transmits all of the energyapplied at input 18 to output 19, and

doesnot reflect any energy back to the source, as is well known. This iseffected, briefly, asindicated in Fig. 2. The'input energy dividesequally at the window as indicated by dashed lines Y21 and 22, theenergy 22 experiencing 90 phase advance relative to energy 21. Both arereflected and each is equally divided, that energy passing, through thewindow again experiencing 90 phase advance. The combined energies 23,being 180 apart, completely cancel eachother but the energies 24, beingin phase, reinforceeachother, and constitute the output containing allof the input energy.

In order. tomodulate theenergy asit passes through this directionalcoupler, two identical ferrite rods .26 and 27,,Fig. 1, are insertedthrough two holes in the end plate 14- and protrude equal distancesaxially and. centrally within the two hollow rectangular waveguides 11I;

each has 2 /2 turns of heavy wire, widely spaced, suitable for highfrequencies as, for example 10 to 50 me. p. s. In accordance with goodpractice these coils are somewhat spaced from the rods and somewhatseparated from the end plate 14, as well as from solenoids 31 and 32 thefunctions of which are to be described later. This isolation of-the coilturns by spacingfrom all conductive objects reduces losses by inducedcurrents and by capacitive shunting.

In order to prevent the metal of the waveguides and of the end plate 14from acting as a single short-circuited turn around each of the rods,the end plate 14 is provided with two slots 33 and 34. These slots arecontinued as a slot 36 in the median line of the upper broad side of thewaveguide 11 and as a similar slot 36' in the median line of the upperbroad side of the waveguide 12. These two slots are further continued inthescribed in Principles and Applications of Waveguide Transmission, byG. C.Southworth, on page 201; The plates 37 and 39 are separatedbyspacing insulators thick enough to insulate them electrically from eachother, a space of one thirty-second inch being satisfactory, and aremechanically secured together at their corners by insulating bolts 41.The common side 13 of the waveguides is providedwith-a butt-joint42. Asis well known, choke flanges when thus separated prevent microwave fieldleakage out of the waveguide and also-provide low impedance fortransmission of the microwave field energy through the wave guide.Additionally, this construction prevents short-circuiting of the gaps 36and .36- either by flange 37 or by flange 39.

If modulation is .eifected by impedance increase as in this case, whenthe ferrite rods 26 and 27 are initially unmagnetized, anymagnetizationby the coils 28 and 29 in either sense will produce an impedanceincrease,v thus producing modulation in the waveguides at double thefrequency of the modulating current. It thereforeis necessary tobias theferrite rods by magnetizing them equally to one-half of the maximumamount by which they are to be magnetized during modulation. That is, aconstant magnetization equal .to one-half of thepeak to-peak maximummodulation should be applied. This is;

suitable direct-current bias-being applied tocoils 31 and 32, microwaveenergy isapplied to the left end'18of waveguide 11 and modulationcurrent 'is applied from modulation generator 44 to the two modulatingcoils 28 and 29 equally. To effect this these coils may be con nectedeither in series orin'parallel as depicted, so that the modulations ofthe magnetic induction of thefe'rrite rods 26 and 27 inside thewaveguides, along theirfred ends 26 and 27' therein, respectively, willbe equal in The energies reflected inside the respective waveguides fromthe end plate 14 will then be modulated in accordance with'the magneticinduction modulation, the amount of modulation and hence re? flectedenergy being equal in the two waveguides in all" respects. The energiesreflected toward'the source end lfi'will'then cancel each'other at allinstants exactlyas in the absence of modulation, and the energiesreflected to the output end 19 will then add at all instants exactly. asin the absence of modulation, so, that the modulation z amplitude andphase.

of the output is effected to the same degree as it vis in each of theindividual waveguides at the ferrlte rods. The modulating componentsincluding the ferrlte rod,

modulating coil and biasing coil, may be applied in sev-; eral othermicrowavedevices to. modulate m1crowave.-

In all'cases, by application oftheprinciples energy. exemplified in thedescription of Fig. 1 including placement of the modulating coil outsideof thewaveguide and slotting .of the waveguide wall where the ferriterod passes through it, the microwave modulator is made suitable formodulation at all frequencies without upper limit. When a magnetized,ferriterod causes rotation of a guided microwave field or exerts arotativeeflort upon the field, it is principally that portion of theferrite which.

extends longitudinally in the waveguide which causes the effect orexerts the rotativeeffort. It is not necessary for the ferrite rod to beaxially or even longitudinally.

rod. should preferably extend in other than a transverse direction. Ittherefore is satisfactory to position. the rod obliquely in thewaveguide and this position is convenient when the microwavemodulatorrequires the ferrite rod to be placed in a waveguide whichisnot short-circuited The waveguide may be either round or rectangular,in accordance .withthe modulating method:-

by an end 1 plate.

employed.

An example of a ferrite rod obliquely positioned in a round waveguide isillustrated in Fig. 3. A round waveguide 46 is provided with chokecouplings 47 and 48 at its ends, each coupling comprising a choke flangeand a plane flange separated by insulating spacers, as before described,so as to prevent electrical current conduction between the flanges whilepermitting field transmission in the waveguide passing through them. Aferrite rod 49 is positioned obliquely in the round waveguide tointersect its axis at a 45 angle, and is provided with a direct-currentbias coil 51 and a modulating coil 52, the latter being depicted as amultilayer solenoid but in each case being designed for the frequency tobe used as pre viously described. The round waveguide is slotted by slot53 throughout its length along a line including the entrance point ofthe ferrite rod 49, so as to prevent metallic continuity around the rod.The slot is continued as slots 55 and 55' in the flanges 47 and 48terminating the round waveguide in the manner previously described.

in order to prevent radiation from slot 53 and also to preventinterference with microwave current flow in the inner surface of thewaveguide, the slot 53 is preferably protected by a choke coarning. Sucha choke coaming is depicted in Figs. 4 and 5. In Fig. 4 the ferrite rod49, shown sectioned, passes through the hole 54 in the wall of thewaveguide. The hole 54 is extended longitudinally in two narrow slot 53and 53" protected by the choke coaming 56 and the plain coaming 57,these coamings being illustrated in section in Fig. 5. The coaming 56contains a quarter-wave slot 58 which presents high impedance at theexternal lips 59, thus preventing radiation, and the coaming contains anadditional slot 61 having such length that the inner lips 62 areone-half wavelength from the closed end of slot 58, thus presenting lowimpedance at the inner surface of the waveguide.

What is claimed is:

l. A microwave modulator comprising, a waveguide section, a ferrite rodextending through a wall of said waveguide section having one endthereof positioned internally of said waveguide section and extendinglongitudinally therealong with its other end projecting externally ofsaid waveguide section, a slot interrupting the peripheral wall of saidwaveguide section extending along the entire longitudinal lengththereof, choke coupling means for said waveguide section, said chokecoupling means comprising a pair of flanges insulated from each other,one of said flanges being atlixed to an end of said waveguide sectionand being provided with a slot forming an extension of the slot in thewall of said waveguide section, and means coupled to the end of the rodexterior to said waveguide section for modulatingly magnetizing saidrod.

2. A microwave modulator in accordance with claim l in which saidmicrowave guide is rectangular.

3. A microwave modulator in accordance with claim 1 in which saidmicrowave guide is round.

4. A microwave modulator comprising, a waveguide section, a ferrite rodextending through a wall of said waveguide section having one endthereof extending into the interior thereof at an acute angle asrespects the longitudinal axis thereof with its other end projectingexteriorly of said waveguide section, a slot extending through the wallof said waveguide section and extending along the entire length thereofthrough the point of juncture of said rod and said waveguide section,choke couplings for each end of said waveguide section, each of saidchoke couplings comprising a pair of flanges insulated from each other,one of each of said pair of flanges being aflixed to a respective end ofsaid waveguide section and being provided with a slot forming extensionsof the slot in said waveguide section, and means coupled to the end ofthe rod exterior to said waveguide section for modulatingly magnetizingsaid rod,

5; A microwave modulator in accordance with claim 4 in which saidmicrowave guide is round and in which said slot is protected by a chokecoaming.

6. A microwave modulator comprising, an input microwave rectangularwaveguide adapted to be energized at one end by microwave energy fortransmission therethrough in the TE mode, an output microwaverectangular waveguide positioned in juxtaposition to said inputwaveguide and parallel thereto, one narrow side of each waveguide beingcommon to both waveguides, a window in said common side adapted totransmit one half of microwave energy in either waveguide to the other,a

' conductive plate closing and short-circuiting the other end of saidinput waveguide and the adjacent end of said output waveguide, twoferrite rods, each passing through said plate axially into a respectiveone of said input and output waveguides, said plate and said waveguidesbeing provided with slots extending radially from said respective rodsthereby interrupting the circumferential conti nuity of the conductiveplate and waveguide around each of said two rods, means magneticallybiasing said rods in equal amounts and senses and means modulatinglymagnetizing said rods in instantaneously equal magnitudes,

frequencies and phases.

7. A microwave modulator comprising, a rectangular waveguide sectionhaving a conductive plate closing one end thereof, a ferrite rodprojecting through said conductive plate with one end thereof extendinginteriorly and longitudinally along said waveguide section, a slot cutthrough a broad wall of said waveguide section along the entire lengththereof and through said conductive plate to the juncture of said rodand plate, a choke coupling for the other end of said waveguide section,said choke coupling comprising a pair of flanges insulated from eachother with one of said flanges affixed to the end of said waveguidesection and provided with a slot forming an extension of the slot insaid waveguide section, said rod having an end projecting exteriorly ofsaid waveguide section, a coil surrounding the end of said road exteriorto said waveguide section, and a variable current supply for said coil.

References Cited in the file of this patent UNITED STATES PATENTS2,402,948 Carlson July 2, 1946 2,671,884 Zaleski Mar. 9, 1954 2,728,050Van de Lidt Dec. 20, 1955

